In a wireless communication system which uses multiple carriers, such as an orthogonal frequency division multiple access (OFDMA) or a single carrier-frequency division multiple access (SC-FDMA), radio resources are a set of continuous sub-carriers and are defined by a time-frequency region on a two-dimensional sphere. A time-frequency region in the OFDM or OFDMA scheme is a rectangular form sectioned by time and sub-carrier coordinates. In other words, one time-frequency region could be a rectangular form sectioned by at least one symbol on a time axis and a plurality of sub-carriers on a frequency axis. Such a time-frequency region can be allocated to an uplink for a specific user equipment (UE), or a base station can transmit the time-frequency region to a specific user equipment in a downlink. In order to define such a time-frequency region on the two-dimensional sphere, the number of OFDM symbols and the number of continuous sub-carriers starting from a point having an offset from a reference point should be given.
An evolved universal mobile telecommunications system (E-UMTS) which is currently being discussed uses 10 ms radio frame comprising 20 sub-frames. Namely, a sub-frame has a length of 0.5 ms. A resource block comprises one sub-frame and twelve sub-carriers, each of which is 15 kHz. One sub-frame comprises a plurality of OFDM symbols and a part (for example, first symbol) of the plurality of OFDM symbols can be used for transmission of L1/L2 control information.
FIG. 1 illustrates an example of a structure of physical channels used in the E-UMTS. In FIG. 1, a sub-frame comprises an L1/L2 control information transmission region (the hatching part) and a data transmission region (the non-hatching part).
FIG. 2 illustrates a general method of transmitting data in the E-UMTS. In the E-UMTS, a hybrid auto repeat request (HARQ) scheme which is one of data retransmission schemes is used to improve throughput, thereby enabling desirable communication.
Referring to FIG. 2, the base station transmits downlink scheduling information (hereinafter, referred to as ‘DL scheduling information’) through DL L1/L2 control channel, for example, a physical downlink control channel (PDCCH), to transmit data to a user equipment in accordance with the HARQ scheme. The DL scheduling information includes user equipment identifier (UE ID) or group identifier (group ID) of user equipments, location and duration (resource assignment and duration of assignment) information of radio resources allocated for transmission of downlink data, modulation mode, payload size, transmission parameters such as MIMO related information, HARQ process information, redundancy version, and new data indicator. The location of the radio resources can be expressed by physical resource block (PRB) information. One PRB is a minimum unit of channel resources allocated to the user equipment, and PRB information includes frequency and/or time interval information allocated to the user equipment.
The DL scheduling information can be transferred through the DL L1/L2 control channel even when retransmission is performed. In this case, corresponding information may be changed depending on channel status. For example, if channel status is better than that of initial transmission, the information can be transmitted at a high bit rate by changing the modulation mode or the payload size. By contrast, if channel status is not good, the information can be transmitted at a bit rate lower than that of initial transmission.
The base station transmits user data to the user equipment by using transmission parameters included in the DL scheduling information through channel resources allocated through the DL scheduling information, for example, a physical downlink shared channel (PDSCH) which is a physical channel. The user equipment monitors the PDSCH per transmit time interval (TTI) to identify its DL scheduling information and then receives user data transmitted from the base station by using the DL scheduling information. The user equipment can identify that corresponding scheduling information is transmitted thereto by using UE ID or group ID included in the DL scheduling information.
In the E-UMTS system, a maximum frequency bandwidth used in a downlink is 20 Mhz. At this time, if there are data in a sub-frame, wherein the data are to be transmitted from the base station to the user equipment or are to be received from the user equipment, transmission of downlink or uplink scheduling information for data transmission or reception through the entire radio resources or the entire frequency bandwidths may cause a waste of radio resources. Actually, since the amount of downlink or uplink scheduling information is less than that of information that can be transmitted through the entire radio resources, the related art may cause a waste of radio resources.
Since the user equipment should monitor the entire L1/L2 control information region per sub-frame to receive scheduling information transmitted thereto, a data processing procedure of the user equipment becomes complicated, whereby resources of the user equipment are wasted.